1 / Place the salmon fillets in an oven dish. Pour over the beer and leave to marinate for three hours.
2 / Trim the leeks, rinse under running water and chop finely. Melt the butter in a casserole dish and simmer the leeks with pepper, salt and thyme on a low heat for 15 minutes.
3 / Pour the marinade into a saucepan, adding pepper and salt. Bring to the boil, and poach the salmon fillets in the liquid for 10 minutes on a medium heat, gently turn them over and continue poaching for another 5 minutes.
4 / Remove the fish and arrange it on a serving dish, where it can be kept hot. Reduce the sauce using a high temperature setting. Add the cream and thicken further until the sauce is creamy. Remove from the hob and add the chopped parsley and garden cress. Pour the sauce over the fish and arrange with the leeks and boiled potatoes.